I can answer 1/2 of this question.
Query calls (to fetch data without persisting state mutations in a canister) are currently free. So downloading 1GB or 1TB is $0. There is a proposal for query charging, so this could change in the future.
Much of this depends on the logic that you use to upload/store the image and the size of each image. I’d imagine @peterparker might have some data and estimates from the work he’s done on Juno.